1. Field of the Invention
The present invention relates to control of the amount of energization of an alternating-current motor. In particular, it relates to a motor controlling device that controls the amount of energization of an alternating-current motor by turning on and off switching elements forming an H bridge.
2. Description of the Related Art
Conventional motor controlling devices for a three-phase alternating-current motor, which is a typical alternating-current motor, are configured as shown in FIG. 7 and control the amount of energization of the three-phase alternating-current motor by supplying current command values balanced among the three phases (the U phase, the V phase and the W phase) to armatures 230, 231, 232. Specifically, a PWM circuit 241 and a base drive circuit 240 control the amount of energization by the PWM control that involves changing the ratio between the directions of energization of the armatures 230, 231 and 232 by a direct-current power supply 210 for each control cycle.
In this case, if the amount of energization of the U-phase armature 230 is 100% of the rated current, for example, the amount of energization of the V-phase armature 231 and the W-phase armature 232 is 50% of the rated current. Thus, compared with U-phase switching elements 220 and 221, V-phase switching elements 222 and 223 and W-phase switching elements 224 and 225 have a low current utilization ratio. In addition, when the rotational speed of the alternating-current motor 200 is low, a large current flows through the part of one phase for a long time, and therefore, there is a problem that the heat generated by the switching elements and armature increases.
Thus, as shown in FIG. 8, there has been proposed a motor controlling device that has H bridge circuits 310, 311 and 312 provided for three phase armatures 301, 302 and 303 of a three-phase alternating-current motor 300, respectively, which enable independent control of the amount of energization of each of the armatures 301, 302 and 303, and adds an offset current to the current command values balanced among the three phases in such a manner that the combined spatial vector for the three phases is zero (see Japanese Patent Laid-Open No. 2004-135437, p. 4-5, FIGS. 1 to 13, for example).
Since this motor controlling device adds the offset current, currents not balanced among the three phases can be supplied to the armatures 301, 302 and 303. Therefore, the motor controlling device can compensate for unbalanced amounts of energization of the armatures, thereby reducing the heat generated by the switching elements and the armatures described above. However, this motor controlling device is still based on the use of the current command values balanced among the three phases.
In the case where the amount of energization of each armature is PWM-controlled as described above by controlling the current command values balanced among the three phases, if the current command values are zero, FET1 to FET4 of the H bridge circuits 310, 311 and 312 are turned on or off as shown in FIG. 9(a) so that the average of the amounts of energization is zero.
In this case, the ratio between the directions of energization in each control cycle A (t30-t32, t32-t34, t34-t36, t36-t38) is 1:1 (duty=50%), and as shown at the bottom of FIG. 9(a), a wasted current shown by the shaded areas flows through the armatures 301, 302 and 303 of the alternating-current motor and the switching elements (FET1 to FET4) of the H bridge circuits 310, 311 and 312. Then, heat is generated in proportion to the square of the wasted current.
Furthermore, in the case where the current command values balanced among the three phases increase, as shown in FIG. 9(b), the average β of the amounts of energization increases, and a wasted current shown by the shaded areas at the bottom of FIG. 9(b) is consumed, and therefore, the armatures of the alternating-current motor and the switching elements of the H bridge circuits generate heat in proportion to the square of the wasted current. The same holds true for the case where the current command values balanced among the three phases decrease.
The present invention is devised in view of such circumstances, and an object of the present invention is to provide a motor controlling device that reduces heat generated by an armature and switching elements when an alternating motor is activated.